The optical element having the aspherical surface is, for example, an optical component such as an optical lens or an optical mirror used in optical systems, such as telescopes used in astronomy, or optical systems, used for imaging structures of a mask such as a reticle, onto a radiation sensitive substrate, such as a resist, in a lithographic process. The success of such an optical system is substantially determined by the precision with which the optical component can be machined or manufactured to have a target shape. In such manufacture, it is necessary to compare the shape of a surface of the machined optical component with its target shape, and to determine differences between the machined and target surfaces. The surface is then further machined at those areas where the differences between the machined and target surfaces exceed e.g. a predefined threshold.
In order to precisely measure the shape of an optical surface, an interferometer apparatus is commonly used. Examples of conventional interferometer apparatuses are disclosed in U.S. Pat. No. 4,732,483, U.S. Pat. No. 4,340,306, U.S. Pat. No. 5,473,434, U.S. Pat. No. 5,777,741, U.S. Pat. No. 5,488,477, which documents are incorporated herein by reference.
The conventional interferometer apparatus for measuring an aspherical optical surface typically includes a light source for generating a beam of measuring light having flat or spherical wavefronts and arranging a flat reference surface or spherical reference surface, respectively, in the beam of measuring light for generating a reference beam reflected from the references surface. Since flat or spherical reference surfaces may be manufactured and independently tested with a high accuracy, the reference beam will have substantially flat or spherical wavefronts. The beam of measuring light will further traverse a null lens arrangement or compensator which transforms the flat or spherical wavefronts of the beam of measuring light to an aspherical wavefront corresponding to the optical surface to be tested. Background information relating to null lens arrangements or compensators is available e.g. from Daniel Malacara, “Optical Shop Testing”, 2nd Edition, John Wiley & Sons Inc., 1992, chapter 12. The aspherical wavefronts will be reflected from the optical surface to be tested, and traverse the null lens arrangement such that the reflected aspherical wavefronts will be transformed to closely spherical or flat wavefronts which are superimposed with the wavefronts of the reference beam to generate an interference pattern on a detector. From the interference pattern it is possible to determine the shape of the aspherical optical surface relative to the reference surface. One source of errors in such determination results from a limited accuracy with which the optical effect of the null lens arrangement is known.